QDV vs a Mechanical chamber sealing valve

iknowmy3tables · Fri Nov 13, 2009 5:28 pm

I was looking at the design of Technician's QDV
and I think it's safe to describe the valve as a mechanical barrel sealing piston valve, and I was trying to compare it to another design that I recall but can't find that I would describe as a mechanical chamber sealing valve, you can see the diagram below

GGDT isn't built for mechanical valves and and give me a lot of problems, right now to approximate I tried to puting in the vent diameter to be .1 smaller than the piston diameter, and for the QDV I put in the seat to be .1 smaller as well, GGDT forced me to increase pilot volume (I don't know why), but the crude comparison said they are the same performance wise.

this is as confusing as the day I learned barrel-sealing valves opened faster than chamber-sealing valves, even worse I'm starting to question some of that now

so please help me get my facts straight the better opening speed of barrel sealing valves is because the vacuum of the barrel keeps the piston forward while the pilot depressurizes even more until the piston gives way with a greater pressure difference pushing it. however is it isn't necessarily true that the larger you make the seat (with the piston diameter constant) the faster the valve opens

I'm quite confused now surely a mechanical chamber sealing piston valve should open faster (though is more difficult to build because of the latch that will have to hold the full force of the piston under pressure and minimize friction) than a QDV

I think I need help reestablishing a new definition as to under what circumstances are barrel sealing valves faster than chamber sealing valves

boyntonstu · Fri Nov 13, 2009 5:43 pm

The O rings seal the bottom.

What seals the top?

A double set of O rings comes close to Tech's QDV.

Fri Nov 13, 2009 5:45 pm

Been thinking about this particular type of valve.I would use an adjustment screw to make sure it is squeezed hard enough against the seal and a lever to both cock it, and trigger it...

It might be a very fast valve...but I like it for my own reasons..

Think Benjamin pump...filling directly into the chamber..no pilot exhaust and a silencer shroud over the barrel...

That would be a very practical cannon..

spudtyrrant · Fri Nov 13, 2009 5:45 pm

iknowmy3tables wrote:I was looking at the design of Technician's QDV
and I think it's safe to describe the valve as a mechanical barrel sealing piston valve, and I was trying to compare it to another design that I recall but can't find that I would describe as a mechanical chamber sealing valve, you can see the diagram below

uhhh.... that isn't the best way to describe tech's valve, tech's valve is neutral it won't go either way until it is activated, the best way i could think to describe it would be a neutral mechanically activated toolie valve since it doesn't really seal against the barrel, it's more of a piston valve with a high CF.
plus his valve does technically seal the chamber lol

Technician1002 · Fri Nov 13, 2009 7:18 pm

To model my valve, I lie to GGDT. I tell it the piston OD is about .1 to .2 inches larger than the valve seat.

The main difference between a barrel sealer and chamber sealer is determined on what is on the face of the piston. Is it a barrel? Is it a chamber? With that, the QDV is a barrel sealer. The face of the piston faces the barrel, not a chamber under pressure.

GGDT does not like a valve to have the piston face and the piston diameter the same diameter. It thinks it won't move as it is assuming the only motion is from air pressure, not a mechanical pull. Lying to GGDT makes it work somewhat, but GGDT is then calculating a slow start to the opening due to air, not the mechanical stroke the rod gives it. In reality, my valve starts opening faster than GGDT models an air only valve.

Chamber sealers have high pressure on the face of a piston. This will require a high pressure in the pilot area or a mechanical link to hold it closed. Due to that, when fired, the valve opens relatively early while the pilot is under high pressure. The chamber begins to quickly vent and drop in pressure. This reduces the force opening the valve and it may re-close if the pilot valve is not huge to keep ahead of the rate the chamber vents out the barrel. Many sprinkler valves are of this configuration where the inlet presses on the center of the diaphragm. A slow pilot causes honking.

The QDV as a barrel sealer (pressure on the piston side) has no pressure in the "Pilot area" so it has the advantage of not requiring a large pilot valve to quickly vent this area. It's already vented. When the valve cracks open (quickly by the rod impact) the chamber dumps onto the face and blows the valve open with pressure between the piston an projectile without any resistance from pressure behind the piston. It opens fast.

Without a linkage and sear or pilot pressure needing vented, the QDV opens faster than either a traditional chamber or barrel sealer. There is less resistance to it opening and less pressure slowing it. It does tend to beat the tar out of bumpers.

On the drawing the mechanical link is often replaced by air and a pilot valve. The Supah valve is constructed in this configuration.

Note the inlet is on the face of the piston. This makes it a chamber sealer.

I hope this helps.

iknowmy3tables · Sat Nov 14, 2009 12:44 pm

it may re-close if the pilot valve is not huge to keep ahead of the rate the chamber vents out the barrel. Many sprinkler valves are of this configuration where the inlet presses on the center of the diaphragm. A slow pilot causes honking.

that aplies to pistons and sprinkler valves of both configurations,

what I am saying is that the last convention I recall on barrel sealing piston valves vs chamber sealing piston valves seamed to declare barrel sealing to be always faster than chamber sealing equivalents,
which would have to make the assumption that:
(the force, in a barrel sealing piston, opening the piston, due to air pressure,) = ((the entire cross sectional area of a piston) x (the pressure that the chamber is filled to))-((the entire cross sectional area of a piston)x(the pressure in the pilot))
which is not true because of the area of the seat is initially under a vacuum

while pressure may affect the seat area once opened (and this pressure is what the QDV uses to open) the average pressure on this area is significantly less than the pressure in the chamber because of the rapid depressurization of air exiting though the barrel. in other words the cross sectional areas under pressure initially contribute more opening force per square inch than cross sectional area that is not initially under preassure

Technician1002 · Sat Nov 14, 2009 1:40 pm

which is not true because of the area of the seat is initially under a vacuum

This is commonly tossed about and poorly understood. The start of motion in a barrel sealer is often delayed and mistaken for slow performance due to the initial opening force being quite low. The flow through the mostly closed valve is also quite low, but avalanches in force as it does open. This in comparison of a chamber sealer which opens sooner (as soon as the pilot pressure releases it) will start to move sooner, but unlike a barrel sealer that increases the force as it opens, as the chamber pressure drops, the opening force drops as the valve opens, reducing or even reversing the overall force on the piston. This happens unless you come up with a really huge and fast pilot valve to keep the pilot pressure dropping faster than the chamber pressure drops.

The advantage of chamber sealers and this reducing force is they are much less likely to break things. I think the supah valve is built this way to reduce breakage.

jackssmirkingrevenge · Sat Nov 14, 2009 1:43 pm

*cough*

dewey-1 · Sat Nov 14, 2009 1:49 pm

JSR; You have to do something about that smoker's cough you have!

All you ever show is: cough!

Sat Nov 14, 2009 3:36 pm

The start of motion in a barrel sealer is often delayed and mistaken for slow performance due to the initial opening force being quite low. The flow through the mostly closed valve is also quite low, but avalanches in force as it does open.

Wich is why my piston valves have a diameter very close to the seat size.

I want to use this effect to the maximum.
Having a very small surface area exposed to the chamber means the pilot has to drop very low before movement starts.When the piston starts moving the entire cross section is exposed, to wich the pilot now has no answer...

Never the less...I would love to give the sear thing a try once..

dewey-1 · Sat Nov 14, 2009 3:51 pm

@BTB;

I too have been pondering a spring loaded QDV with an elbow catch as the triggering mechanism. Something similar to this;

iknowmy3tables · Sun Nov 15, 2009 1:52 am

jackssmirkingrevenge wrote:*cough*

right I remember that one, but the one I recall was different, it had two separate chambers that were connected to the piston valve via ball valves to allow two shots one from each of the chambers

Brian the brain wrote:
The start of motion in a barrel sealer is often delayed and mistaken for slow performance due to the initial opening force being quite low. The flow through the mostly closed valve is also quite low, but avalanches in force as it does open.
Wich is why my piston valves have a diameter very close to the seat size.

I want to use this effect to the maximum.
Having a very small surface area exposed to the chamber means the pilot has to drop very low before movement starts.When the piston starts moving the entire cross section is exposed, to wich the pilot now has no answer...

Never the less...I would love to give the sear thing a try once..

yes yes this is true and I already acknowledged the delayed avalanche when I said

while pressure may affect the seat area once opened (and this pressure is what the QDV uses to open)...

however you have not told me why the force per area that is exposed after the valve opens is equal to the force per area that is exposed the entire time,
my argument is that once opened the air is rapidly depressurizing much like the rest of the barrel section which we typically engineer with much much lower strength standards than the chamber.
anyways to say that the two types of area were equal in energy they applied would mean that an ideal mechanical chamber sealing piston like the one in the above diagram has the same performance as an ideal QDV or more because of the force of the pull, and come to think of it if you accept that than you must accept that initial forces have nothing to do with performance, and whether this is a significant factor may be provable with a simple test:
Technician if you actuated the QDV by pulling the knob quickly vs really gently do you notice a change in performance?

dewey-1 wrote:@BTB;

I too have been pondering a spring loaded QDV with an elbow catch as the triggering mechanism. Something similar to this;

no one mentioned a spring loaded valve

Technician1002 · Sun Nov 15, 2009 2:35 am

anyways to say that the two types of area were equal in energy they applied would mean that an ideal mechanical chamber sealing piston like the one in the above diagram has the same performance as an ideal QDV or more because of the force of the pull, and come to think of it if you accept that than you must accept that initial forces have nothing to do with performance, and whether this is a significant factor may be provable with a simple test:
Technician if you actuated the QDV by pulling the knob quickly vs really gently do you notice a change in performance?

The force on the piston to move it is a balance of forces from the front and back. A traditional piston valve begins to open as the pilot pressure drops so the force on the rear of the piston is lower than the force on the chamber exposed area of the face of the piston. This means the piston begins to open while the pressure in the pilot is still decreasing. A QDV on the other hand begins to open while the pilot area pressure is zero. If the mechanical sear is used instead of pilot pressure, the addition of the mechanical rod adds mass to the piston.

The force on the face of a piston on a QDV and regular chamber sealer is the same as the piston opens. What is not the same is the pressure in the back of the piston.

On pulling the valve open at high pressure, there is no difference in power as far as I can tell on a gentle or hard pull. With an empty barrel or very lightweight projectile it is possible to crack the valve and not generate enough pressure to pop it open. I have found it possible if I try hard to crack the valve and not generate enough pressure with an empty barrel to pop the valve open. This misfire is very rare and takes effort to just knock the valve enough to get it to bleed but not pop at higher pressure.

At very low pressure, then a gentle pull verses a yank does make a substantial performance change. In launching eggs, the pressure is too low to pop the valve open hard. This is in the pressure range most piston valves simply won't open, so yes it does outperform piston valve in that low range. Most piston valves don't work very well or at all below 10 PSI. I've operated using under 5 PSI.

jackssmirkingrevenge · Sun Nov 15, 2009 5:52 am

Brian the brain wrote:...which is why my piston valves have a diameter very close to the seat size.

I want to use this effect to the maximum.
Having a very small surface area exposed to the chamber means the pilot has to drop very low before movement starts.When the piston starts moving the entire cross section is exposed, to which the pilot now has no answer.

This only works if you have a tight/o-ringed piston and large pilot valve. For those without these ideal conditions it's advisable to have the piston diameter considerably larger than the barrel.

You have to do something about that smoker's cough you have!

It's not the smoking that causes it, but having to stay outside in the cold wet smoking area!

Technician1002 · Sun Nov 15, 2009 6:04 am

jackssmirkingrevenge wrote:
This only works if you have a tight/o-ringed piston and large pilot valve. For those without these ideal conditions it's advisable to have the piston diameter considerably larger than the barrel.

In smaller sizes, a lapped finish works fine. A large pilot valve is not required. With low leakage (small eq port) a small valve works fine.

The Mouse Musket did not use rings and used a 1/4 inch ball valve for a trigger. It has an oversize valve seat for a close piston OD to valve seat diameter ratio by using an o ring on the outside of the breech of the barrel. The piston OD is 1.5 inch and the seat is 1.25 inch. The seat is only a 1/4 inch smaller.

Even with slow trigger operation it provides a wonderful HisssBoom. I've posted the audio file and Audacity screenshots in earlier posts. This is using no o rings, a small 1/4 inch trigger valve and slow manual opening of the trigger. This is why the hiss starts small and builds slowly. It then decreases again as the pilot pressure drops to the firing pressure just before the bang.

QDV vs a Mechanical chamber sealing valve

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